Energetic Electron Measurements Research Articles

Simultaneous measurements of energetic protons and electrons in the distant magnetosheath, magnetotail, and upstream in the solar wind

Measurements of energetic protons (Ep > 0.29 MeV) and electrons (Ee ≳ 0.22 MeV) in the magnetopause/magnetosheath region of the tail and simultaneously upstream from the bow shock in the solar wind using data from JHU/APL instruments on the IMP‐7 and IMP‐8 spacecraft during December 9 to 14, 1974 are reported. Similar fluxes of protons and electrons exhibiting extreme time variation were found to be present simultaneously in the solar wind and the distant (≳ 30 Re) magnetopause/magnetosheath during this period. Protons and electrons were streaming in the antisolar direction in the magnetosheath, with the protons exhibiting much larger tailward‐to‐sunward anisotropies (maximum values of ∼ 700∶1 compared to ∼ 9∶1 for protons and electrons, respectively). The ratios of energy densities of protons‐to‐electrons ranged from ∼ 500 to ≳ 2000 in the magnetosheah, during selected time intervals. Protons and electrons upstream were moving sunward away from the bow shock. Additional measurements on September 11‐12, 1975 in the magnetosheath and inside the magnetotail show proton and electron intensities to be much larger (up to ∼ × 10) inside the magnetotail. The observations are interpreted as a temporal rather than spatial phenomenon, the source of the particle bursts being inside the magnetosphere. It is concluded that there is no connection between the presence of energetic particles in the tail magnetopause/magnetosheath and the postulated energy release via field line merging at the subsolar magnetopause.

Rocket measurements of ULF electric fields and precipitating particles in the aurora

The electric component of ULF waves with a characteristic period of 1.5-3 s and an amplitude of tens of mV m has been observed with a double probe on a rocket launched from Andöya in Norway into an aurora. The polarization of the waves was right and left handed nearly circular as well as nearly linear during different intervals of the flight. These measurements are compared with ELF and energetic electron measurements on the rocket and ULF measurements on the ground. Possible interpretations involving generation of the ULF waves by proton cyclotron resonance in the magnetotail or by streaming ions or electrons in the magnetosheath are outlined.

Mid-LatitudeERegion: An Examination of the Existence of a Corpuscular Source

We present new data from two rocket flights and examine the published evidence for the existence of a corpuscular source of ionization at E region heights. The rockets were launched from White Sands Missile Range (magnetic latitude of approx.43degree) to provide measurements of energetic electrons and protons between 40 eV and 5.5 keV in the E region below 120 km. A daytime flight (solar zenith angle chi of 58degree) launched during a geomagnetically disturbed period (Kp=5=) provided measurements of a small flux of electrons which exhibits a strong altitude dependence below 500 eV but essentially none above that energy. The observed electron energy flux at 116 km is approx.1.8 x 10/sup 3/ erg/cm/sup 2/ s. We interpret these to be photoelectrons. During the nighttime flight we observed an energy flux which was less than 10/sup 5/ erg/cm/sup 2/ s. Upper limits for proton fluxes are given. The electron flux values contrast sharply with much higher values reported by some other investigators. The implications of and some possible reasons for the differences are discussed. After reviewing our data and the relevant literature we conclude that (1) data obtained from one location must be carefully interpreted with regard to implications about themore » general ionosphere, since a real geographic variability exists: (2) since we observe no significant fluxes of energetic particles in a geomagnetically disturbed ionosphere, even in the presence of fluctuating nighttime E region ionization, the presence of such fluctuations cannot be taken as evidence of precipitating particles: and (3) many reports of corpuscular radiation contain high flux values which are not supported by observations of 391.4-nm radiation from N/sub 2/. (AIP)« less

Jupiter Revisited: First Results from the University of Chicago Charged Particle Experiment on Pioneer 11

During the December 1974 Pioneer 11 Jupiter encounter our experiment provided measurements of Jovian energetic protons and electrons both in the magnetic equatorial zone and at previously unexplored high magnetic latitudes. Many of the observations and conclusions from the Pioneer 10 encounter in 1973 were confirmed, with several important exceptions and new findings. We report evidence from Pioneer 11 for protons ( approximately 1 million electron volts) of Jovian origin in interplanetary space. In the outer magnetosphere particle intensities at high magnetic latitudes were comparable to those observed in the equatorial zone, and 10-hour variations in particle intensities and spectra were observed at both high and low magnetic latitudes. Therefore, confinement of particles in the outer magnetosphere to a thin equatorial magnetodisc is adequate neither as a description of the particle distribution nor as a complete explanation of the 10-hour variations. Pioneer 11 data support a model in which the intensity varies with a 10-hour period in phase throughout the sunward side of the magnetosphere and is relatively independent of position within the magnetosphere. Transient, highly anisotropic bursts of protons with energies of approximately 1 million electron volts observed near the orbit of Ganymede suggest local acceleration in some regions of the magnetosphere. In the inner core where particles are stably trapped, a maximum in the high-energy nucleonic flux was again found, corresponding to the Pioneer 10 maximum at approximately 3.4 Jupiter radii (R(J)), which is apparently a persistent feature of, the inner radiation zone. In addition, Pioneer 11 data indicate two more local maxima in the nucleonic flux inside 3.4 R(J), one of which may be associated with absorption by Amalthea, and a maximum intensity at 1.9 R(J) more than 20 times that at 3.4 R(J), The flux of relativistic electrons reached a maximum on the magnetic equator at 1.8 R(J), only slightly less its closest approach at 3.1 R(J).

Current system in an auroral substorm

An instrumented sounding rocket was launched at 0040 LT into the expansive phase of an auroral substorm on March 16, 1973, from Poker Flat, Alaska. This report describes coordinated measurements of electric field, magnetic field, and energetic electrons. By comparing the observed magnetic field with that predicted by model currents, a current system, which includes field-aligned currents, is deduced. The model indicates three currents: a southward current of 14.37 µA/m², a westward current of 22.5 µA/m², and two field-aligned currents of 5 µA/m². A southward electric field of 60 mV/m identifies the southward current as a Pedersen current and the westward current as a Hall current. The field-aligned current measured from particle detectors shows a structure similar to that in the magnetic model. The total field-aligned electron current measured by particle detectors agrees within 10% of the current model.

Prolonged tailward flow of plasma in the thinned plasma sheet observed atr≈ 18REduring substorms

Measurements of plasma and energetic electrons made during substorms by Vela satellites within ≲1 RE of the magnetotail neutral sheet are reported. Rapid antisunward flow of plasma is observed to start at the onset of the expansion phase of a substorm (i.e., at breakup) and to continue until the expansion phase ends and substorm recovery begins, as is indicated by auroral zone magnetograms. When substorm recovery begins, the plasma flow reverses direction, and rapid sunward flow begins. Fluxes of energetic electrons are low, and the temperature of the plasma is reduced during the interval of antisunward flow. The lowest fluxes (i.e., complete dropouts) of energetic electrons are observed just before the reversal of plasma flow to the sunward direction. After the reversal the energetic electron flux intensity rises rapidly to high values as the plasma sheet recovers. These observations are regarded as added evidence that a neutral line forms between the earth and the Vela orbit (r ≈ 18 RE) at substorm breakup and then moves suddenly to distances beyond the Vela orbit when substorm recovery starts. Throughout the expansive phase there exists a thin residual plasma sheet beyond the neutral line in which plasma continually flows rapidly downstream into the more distant tail. The particle dropout just preceding the flow reversal and plasma sheet recovery is thought to be the signature of the passage of the neutral line near the Vela satellite as it moves rapidly outward in the tail. Spin modulation of the proton flux observed continuously for about 1 hour during very quiet conditions preceding one substorm may constitute a direct measurement of the proton component of the cross-tail electric current that maintains the taillike configuration of the magnetotail field.

Location of the electron acceleration region in solar flares

Observations of impulsive solar flare X-rays ≳ 10 keV by the OGO-5 satellite and the measurements of energetic solar electrons made with the Explorer-35 and Explorer-41 (IMP-5) satellites during the period March 1968–September 1969 have been analyzed in order to determine the ion density in the X-ray source region as well as the location of the electron acceleration region in the solar atmosphere. If we assume that the efficiency of escape of the accelerated electrons into interplanetary space is ∼ 10−3, the observations are found to be consistent with the following interpretation: (i) the ion density in the X-ray source region varies from event to event and lies between 109 and 1011 ions cm−3 for those events in which the impulsive X-ray emission could be detected; (ii) for those events in which no impulsive emission was detected above threshold, the ion density in the X-ray source was < 109 ions cm−3; (iii) at least in some small solar flares the region where the electrons are accelerated during the flash phase is located in the lower corona.

Strong pitch-angle scattering of energetic electrons in the presence of electrostatic waves above the ionospheric trough region

Simultaneous measurements of energetic electrons and electrostatic waves in the midlatitude trough region indicate that events characterized by strong pitch-angle diffusion in the altitude range 2000-4500 km occur in the presence of ELF electrostatic waves attributed to a current-driven ion cyclotron instability. Scattering by the electrostatic waves is sufficient to account for the strong pitch-angle scattering commonly observed by low-altitude satellites. The resulting precipitation creates a sharply defined, high-latitude boundary for energetic electrons as observed by low-altitude satellites. This boundary is similar to the feature generally referred to as the high-latitude trapping boundary. However, the precipitation boundary occasionally occurs on closed field lines at a significantly lower latitude than the trapping boundary. When present, the precipitation boundary is most sharply defined in the nighttime hemisphere.

Outer-zone electron precipitation

Measurements of energetic electrons (300 kev to 2.3 Mev) within the drift loss cone, of ELF emissions, and of the plasmapause location obtained with instruments aboard the polar satellite OV3-3 (1966-70A) are compared. A knee is found in the L-shell profile of the precipitated fluxes of outer-zone electrons, the flux decreasing toward lower latitudes. The location of the knee correlates with the morning location of the plasmapause but not with the evening location. This finding suggests that outer-zone electron precipitation is influenced by the morning (perhaps minimum) location of the plasmapause. For 3.3≤L≤4.3, electrons are preferentially precipitated in the morning hours after 0600 LT. This precipitation corresponds to the strong morning maximum in the occurrence of whistler-mode ELF emissions. Electromagnetic ELF emissions are often observed near the plasmapause where the maximum flux of precipitated electrons is observed.

Measurements of energetic electrons in the vicinity of the sunward magnetospheric boundary with Explorer 14

Explorer XIV measurements of energetic electron intensities near sunward magnetospheric boundary, noting spikes in transition region