Variations in the high latitude proton trapping boundary associated with polar magnetic substorms

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The pitch-angle distribution of 100-to 300-key protons in the early momingflate afternoon local magnetic-time sector has been studied by using data from the early lifetime of the Esro IB (Bo.reas) satellite. The pitch-angle distroebution is described as a function of invariant latitude for different geomagnetic activity levels. On the nightside, going poleward, the particle fluxes were found to peak at 90 ø, followed by a latitude range with isotropy. On the dayside, two precipitation zones were found during slight disturbances, the poleward zone showing gn isotropic pitch-angle distribution and the equatorward zone showing an anisotropic distribution peaked at 90 ø . During disturbed conditions, the two zones merge and the region of isotropic poetch-angle distribution expands. The particle loss cones were determined and compared with theoretical loss cones in a realistic field model. The phenomenon of energetic protons impinging on the atmosphere may be described by giving, for instance, the flux values and energy and pitch-angle distributions, for different geomagnetic activity levels, as a function of invariant latitude and magnetic local time.In this paper we concentrate on the pitchangle distribution of protons with energy above 100 key. The knowledge of the pitch-angle distribution is necessary to get a quantitative understanding of the processes going on when the energetic particles penetrate the upper atmosphere. To estimate properly the interaction between the incoming particles and the atmosphere, a detailed knowledge of the pitch-angle distribution of the particles is of prime importance. Knowing the pitch-angle distribution, one can also extract information on acceleration processes for the energetic particles and get evidence concerning electromagnetic fields in the precipitation region.Only a few direct measurements of pitchangle distribution of energetic protons in the ionosphere at higher latitudes have been reported. Information of proton pitch-angle distribution obtained by sounding rockets has been reported by, for instance, S•raas and Trumpy [1966] and more recently by Whalen and McDiarmid [1970] and Whalen et al. [1971]. The evidence is generally indicative of a flux maximum perpendicular to the magnetic-field lines or near isotropy over the upper hemisphere. By satellite-borne experiments, Sharp et al. [1967] found isotropy within a factor of 2 for protons with energy above 20 key. Recently Hultqvist et al. [1971] reported on both 'depleted loss-cone' type of anisotropy, to the equatorward edge of the particle zone, and field-aligned anisotropy, to the poleward edge, for ions of below 10 kev of energy. By studying the red shift of auroral hydrogen line profiles, one has found indication of anisotropy in the incident proton flux with a peak 90 ø to the magnetic-field lines. See review by Eather [1967]. Precipitation of protons above 100 key of energy has been studied by using data from the S71C experiment on board the satellite Esro IA (Aurorae) [Aarsnes et al., 1970a]..A slight modification ...

Section: Discussionmentioning

confidence: 98%

“…On the dayside, two zones of protons have been reported for quiet magnetic conditions [Sharp et al, 1967;Lindalen et al, 1971]. The lower zone is probably associated with gradient drifting protons whose origin can be found in the nightside hemisphere.…”

Section: Discussionmentioning

confidence: 99%

Pitch-angle distributions of 100- to 300-kev protons measured by the Esro IB satellite

Amundsen¹,

Søraas²,

Lindalen³

et al. 1972

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“…The magnetospheric trapping boundary has been studied by a number of experimenters using energetic particle data acquired from low-altitude satellites [O'Brien, 1964;Williams and Mead, 1965;McDiarmid and Burrows, 1968;Fritz, 1968Fritz, , 1970Lindalen et al, 1971;Amundsen et al, 1972;Rossberg, 1978]. Various features, such as the drop in counting rate to the background level, the presence of flux enhancements, and the occurrence of isotropy between the trapped and precipitating fluxes, have been used to define the boundary and to study its dependence upon other physical parameters such as local time and geomagnetic activity.…”

Section: Introductionmentioning

confidence: 99%

Studies of the sharply defined L dependent energy threshold for isotropy at the midnight trapping boundary

Imhof¹,

Reagan²,

Gaines³

1979

It has previously been reported (Imhof et al., 1977) that near the midnight trapping boundary the onset of isotropy for electrons and protons often occurs at a sharply defined energy threshold which decreases rapidly with increasing L value. In this paper the addition of energy spectrum measurements to the pitch angle distributions provides new information on the magnetic field configuration in the tail region during these cases. The energy dependent characteristics presented here include the positions and total latitude widths of the transition region from locally trapped to precipitating pitch angle distributions, the intensities and locations of near‐coincident enhancements in the trapped fluxes, and the intensities of electron precipitation at energies below the threshold for isotropy. During geomagnetically disturbed times, isotropy at a given energy tends to occur on lower L shells than it does at quiet times, but the widths of the transition region from locally trapped to isotropic pitch angle distributions show no well‐defined dependence upon geomagnetic activity. No evidence is found that unusually broad transition regions occur at any one characteristic phase of a substorm. At energies below the isotropy threshold the ratios of precipitating to trapped electron fluxes display large variations ranging from less than 10−3 to more than 10−1. Near the trapping boundary, enhancements are often observed in the fluxes of locally trapped electrons having minimum drift altitudes of ∼400 km or less. The L shells at which the maximum fluxes occur often display a rigidity dependence which is similar to that associated with the isotropy phenomenon.

“…The isotropic pitch angle distribution is strongly favoring a local day side source for the protons precipitating at high latitudes. These protons are found at L values well poleward of the trapping boundary [Lindalen et al, 1971], i.e., possibly in a pseudotrapping region. The fact that we are not observing precipitating protons during quiet conditions in the whole latitude interval accessible to pseudotrapped protons but only at a selected narrow poleward band of latitudes, points also to a local day side source.…”

Section: The Energy Spectrum Fits Nicely With Extrapolation From Specmentioning

confidence: 99%

Cleft signature in proton fluxes above 100 keV

Amundsen¹,

Søraas²,

Aarsnes³

1975

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Experimental evidence is presented that the low-energy proton population, which is the signature of the polar cleft region, has a high-energy tail of above 100-keV proton fluxes. The precipitation zone of these higher-energy protons covers the same latitude band as the low-energy particles do and also experiences the same latitudinal displacements during changes in the geomagnetic conditions. The precipitation has an isotropic pitch angle distribution and the measured higher-energy intensities compare favorably with values extrapolated from the spectrum at lower energies. The statistical invariant latitude pattern described by above 100-keV precipitating protons in the magnetic local time span 0006-1800 hours is presented. The measurements by Heikkila and Winningham [1971], Frank [ 1971 ], and Frank and A ckerson [ 1971 ] established that soft particles penetrated to low altitudes through the day side magnetospheric cusps. They also identified these fluxes with the magnetosheath plasma [Hundhausen et at., 1969; Frank, 1970]. More recently, Winningham [1972], Burch [1973], and Yasuhara et al. [1973] have studied and described the latitudinal extent and motion of these low-energy particle fluxes during changing geomagnetic, solar wind, and interplanetary magnetic field conditions. The region is also correlated with light emissions fEather and Mende, 1971;Hoffman and Berko, 1971; Heikkila et al., 1972] and is considered important as an entry for particles into the magnetosphere. Particles of higher energies have also been recorded in these latitude ranges. Electrons above about 50 keV have been observed [Russell et al., 1971; Burrows et al., 1972] but are interpreted to have gradient-drifted from the night hemisphere. Magnetosheath intensities of electrons at these energies are poorly known. Analyses of data obtained by the Esro 1 and Isis 1 satellites have shown that the cusp region also contains significant fluxes of above 100-keV protons [Lindalen et al., 1971; Amundsen et at., 1972; S4raas, 1972; Amundsen, 1973; Wedde et al., 1973; Burrows et al., 1972]. S4raas [1972] has shown this precipitation to be a conjugate phenomenon. Contrary to the electrons, the protons are observed to have an isotropic pitch angle distribution at these latitudes. Lindaten et al. [1971] suggested a local day side source for these highlatitude protons. In the following, attention will be focused on what looks like a clear cusp signature in these above 100-keV proton fluxes. EXPERIMENT The data to be presented in this respect were obtained by a proton spectrometer on board the Esro 1A satellite. The satellite was launched on October 3, 1968, into a near-polar orbit with inclination of 94 ø, initial apogee of 1533 km, and perigee of 258 km. The satellite orbit precessed 0.5 ø per day. The satellite was magnetically stabilized by means of permanent magnets. The proton spectrometer is a joint experiment between the University of Bergen in Norway and the Danish Space Research Institute in Denmark and employs three totally depleted surface b...