Limit on stably trapped particle fluxes determined theoretically and compared with data from Explorer satellites

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Limit on stably trapped particle fluxes

The spatial distribution and magnitudes of field-aligned currents at 800-km altitude over northern high latitudes were determined from Triad magnetometer data recorded at College, Alaska, during the period from July 1973 to October 1974. The characteristics that were determined include the following: (1) Large-scale field-aligned currents are concentrated in two principal areas encircling the geomagnetic pole: region 1, located near the poleward part of the field-aligned current region; and region 2, located near the equatorward part. (2) In region 1 during moderately disturbed conditions (2− ≤ Kp ≤ 4+) the largest current densities occur on the forenoon sector, with currents flowing into the ionosphere (with a peak value of ∼2 µA/m² between 0700 and 0800 MLT), and in the afternoon sector, with currents flowing away from the ionosphere (with a peak value of ∼1.8 µA/m² between 1500 and 1600 MLT). These areas of maximum current density in region 1 are approximately coincident with the location of the foci of the Sqp current system. (3) In region 2 for 2− ≤ Kp ≤ 4+ the largest current densities occur on the night side where auroral electrojets are usually most active, namely, in the evening to premidnight sector, with currents flowing into the ionosphere (with a peak value of ∼ 1 µA/m² between 2100 and 2300 MLT), and in the midnight to morning sector, with currents flowing away from the ionosphere (with a peak value of ∼ 1.3 µA/m² between 0200 and 0300 MLT). (4) The currents in region 1 are statistically larger than the currents in region 2 at all local times except for the sector near midnight (∼2100–0300 MLT), where the region 2 currents are comparable to or slightly larger than the region 1 currents. (5) The region 2 currents in the midnight to morning sector are correlated with the intensity of the westward electrojet, and the region 2 currents in the evening to premidnight sector are correlated with the intensity of the eastward electrojet. (6) The region 1 currents appear to persist, especially on the day side, even during very low geomagnetic activity with a value of current density ≳0.6 µA/m² for Kp = 0. We suggest that the magnetosphere-ionosphere current system which contains field-aligned currents consists of two distinct parts: a permanent part with field-aligned currents in region 1 and the other part having field-aligned currents in region 2, which is an important element of the auroral electrojets.

The amplitude distribution of field‐aligned currents at northern high latitudes observed by Triad

Auroral ionization and excitation by incident energetic electrons

A statistical study has been completed using data from the Defense Meteorological Satellite Program F2 and F4 and the Satellite Test Program P78-1 satellites to determine the average characteristics of auroral electron precipitation as a function of magnetic local time, magnetic latitude, and geomagnetic activity as measured by Kp. The characteristics were determined for each whole number value of Kp from 0 to 5 and for Kp ≥ 6-. At each level of Kp, the high-latitude region was gridded, and the average electron spectrum in the energy range from 50 eV to 20 keV was determined in each grid element. The results show that the high-latitude precipitation region separates into two parts based on the electron average energy. There is a region of relatively hot electrons (EAVE ≥ 600 eV). In this region, the electron average energies are highest on the morningside of the oval. There are two average energy maxima at each Kp level: one postmidnight and the other prenoon. The hot electron region is generally not continuous in MLT but shows a gap between 1200 and 1800 MLT. The hotter electrons carry most of the energy flux into the oval. The energy flux on the nightside increases with Kp, while the level at noon increases with Kp when Kp is small but decreases at higher Kp. The average energy of the hot electrons increases from Kp = 0 to Kp = 3 but is approximately constant for higher Kp. In the second region, average energies are low (EAVE < 600 eV). This region extends from the poleward edge of the hot electron region to the pole. The precipitating electrons in this region carry the majority of the number flux at high latitudes. The largest number fluxes are found on the dayside. The highest fluxes are confined to a crescent-shaped region centered slightly prenoon and extending in MLT over most of dayside and, in some cases, into the nightside. There is a prenoon maximum in the number flux that shows little variability in MLT or in intensity with Kp. The average energy shows a minimum typically between 1100 and 1200 MLT and located toward the poleward edge of the crescent-shaped region of highest integral number flux. We identify the cusp as the region near the average energy minimum and the cleft as the crescent-shaped region.

A statistical model of auroral electron precipitation

The series of polar-orbiting National Oceanic and Atmospheric Administration spacecraft TIROS, NOAA 6, and NOAA 7 have been monitoring the particle influx into the atmosphere since late 1978. This data base has been used to construct statistical global patterns of height-integrated Pedersen and Hall conductivities for a discrete set of auroral activity ranges. The observations of energy influx and “characteristic electron energy” have been binned in a 1° latitude and 2° magnetic local time grid and ordered by an auroral activity index. This index is an estimate of the energy deposited into a single hemisphere by incident particles, a parameter generated directly from the particle observations and, therefore, internally consistent with the statistical patterns that are constructed. An average electron spectrum is associated with each characteristic energy, which enables a height profile of ionization rate in the upper atmosphere to be determined. The use of a pressure coordinate system insures that the normalized ionization rate profiles are independent of atmospheric model parameters. To create the statistical pattern of height-integrated conductivities, however, vertical profiles of atmospheric temperature and composition are assumed, and the ion density enhancements are evaluated from a chemical balance between ion production and recombination based on an “effective” recombination coefficient. The data base can also provide the statistical pattern of particle heating rates and ionization rates over a three-dimensional grid suitable as input to more sophisticated ionospheric and neutral thermospheric codes.

Height-integrated Pedersen and Hall conductivity patterns inferred from the TIROS-NOAA satellite data

Data from about 1100 passes of the polar orbiting satellite Isis 2 have been used to determine average intensities and energies of electrons as functions of magnetic local time and invariant latitude for energies above 150 eV. It is found that at any local time the average energy of electrons (in the range 150 eV to 9.6 keV) is a minimum at or near the latitude of the average position of the trapping boundary for 35-keV electrons but that the deepest minimums occur at local times of 0300 and 1600. It is also found that the highest average fluxes of 150-eV electrons occur at 1500 MLT at the boundary and at 0300 MLT inside the boundary. On the basis of these measurements it is suggested that magnetoshenth particles enter the closed field region of the magnetosphere at all local times and that typically there is some energization associated with this entry. Entry is least likely at local times before noon, and energization is greatest at local times before midnight. It is further suggested that at least two processes, convection and possibly a diffusive mechanism, are involved in the entry of magnetosheath plasma to the magnetosphere.

Average characteristics of magnetospheric electrons (150 eV to 200 keV) at 1400 km

The results of a study of some of the particle properties of the day side cleft are given. A detailed comparison has been carried out of data obtained from the energetic particle detector (EPD) experiment and the soft particle spectrometer (SPS) on the Isis 2 satellite. A significant difference is found in the particle properties prenoon (1000–1200 hours MLT) and postnoon (1400–1600 hours MLT). The electron spectrum is found to be much harder and the proton intensity weaker postnoon than prenoon. It is also found both before and after noon that most of the cleft region, which typically extends about 4° in latitude, can be interpreted as lying on closed rather than open magnetic field lines. The results are discussed in terms of the usual model of the cleft in which magnetosheath plasma enters the magnetosphere directly on open field lines and also in terms of a model in which magnetosheath plasma diffuses across closed lines; neither model seems able to account for the prenoon observations.

Particle properties in the day side cleft

The magnetometer on the Isis 2 satellite is used to study a number of magnetic field perturbations which are interpreted as being due to field-aligned currents whose directions are opposite to the direction usually observed. The perturbations are observed mainly on the dawn side of the earth and can be interpreted as a result of two adjacent and oppositely directed current sheets in which the equatorward current is into the atmosphere and the poleward current out of the atmosphere. These perturbations are observed only at times when the interplanetary magnetic field (IMF) has a strong northerly component, and they are found at latitudes above those usually associated with field-aligned currents. The locations of the magnetic field perturbations are compared with simultaneous particle measurements, and the results are discussed in relation to what might be expected from field line merging models.

Reverse polarity field‐aligned currents at high latitudes

A number of Black Brant II rockets containing various charged-particle detectors have been fired during 1963 and 1964 from Fort Churchill, Manitoba Most of the firings took place at times of auroral events and the rocket instrumentation was designed to study the particles associated with these eventsPitch-angle distributions have been observed for electrons with energies greater than 40 keV, which show varying degrees of isotropy in the pitch-angle range 0° to 90° In no case has a distribution been observed in which the intensity increased towards small angles In some cases electron-intensity changes appear to be correlated with changes in spectrum and angular distribution, while in other cases changes in these quantities do not appear to be relatedThe particle intensity measurements are used along with radio-frequency probe measurements of electron density to infer values for the nighttime recombination coefficient in the D region of the ionosphere

Angular Distributions and Energy Spectra of Electrons Associated with Auroral Events

A rocket experiment to study the charged particles associated with auroral radio absorption is described. A primary electron flux, probably in the form of beams or columns, with a maximum intensity of 2 × 106 particles cm−2 sec−1 was observed. The integral electron energy spectrum could be represented by exp (−E/22 kev) at energies above 30 kev. The angular distribution of the incident electrons was approximately isotropic over the upper hemisphere. Two other types of angular distributions, involving large intensities of upward-moving particles, were observed. The measured electron flux was sufficient to account for the observed radio absorption.Protons in the energy range greater than 500 kev were recorded with a maximum intensity of 6 × 102 particles cm−2 sec−1 sterad−1. The proton intensity was relatively steady and was not correlated with the electron flux either in space or in time. Most of the protons appeared to enter the atmosphere to the south of Fort Churchill.

E. E. Budzinski

Papers

Average characteristics of magnetospheric electrons (150 eV to 200 keV) at 1400 km

Particle properties in the day side cleft

Reverse polarity field‐aligned currents at high latitudes

Angular Distributions and Energy Spectra of Electrons Associated with Auroral Events

Direct measurement of charged particles associated with auroral zone radio absorption