Micropulsations in the morning sector: 1. Ground observations of 10- to 45-second waves Tungsten, Northwest Territories, Canada

Examination of Pc 3, 4 micropulsation wave forms recorded at Calgary in the interval September 2-20, 1969, shows a tendency for signal enhancement to have occurred when the interplanetary magnetic field made a small angle 0xa with the sun-earth line. Scatter plots of hourly Pc 3, 4 amplitudes show a definite trend toward large signals when O xa < 500-60 ø and a corresponding disappearance of significant amplitudes when Oxa > 60 ø, but there was appreciable variability in individual cases. The trend toward the highest amplitudes when Oxa •-0 ø was sharpened under geomagnetically quiet conditions. Power density spectrograms improved the correlation of pulsation strength with low angle in some cases. The results are interpreted as compatible with enhanced probability of Pc 3, 4 excitation when quasi-parallel structure prevails at the subsolar point of the bow shock and as consistent with previous Soviet and Canadian analyses, although certain details of the Russian results are not supported. from the sun and as new as the inference that magnetic substorms are initiated by a southward oriented interplanetary magnetic field (IMF). Recently, attention has been directed to some remarkable correlations between micropulsation characteristics in the 10-to 150-s period range (Pc 3, 4) and solar wind parameters [Troitskaya et al., 1971; Gul'yel'mi et al., 1973; Plyasova-Bakunina, 1972]. One such correlation is manifested in the preferential appearance of Pc 4 when the interplanetary field vector Bsw points either sunward or antisunward, i.e., parallel or antiparallel, to the solar ecliptic X axis and, conversely, in the preferential disappearance of Pc 4 when Bsw is in or close to the solar ecliptic Y-Z plane, i.e., orthogonal to X [Bol'shakova and Troitskaya, 1968; Nourry and Watanabe, 1973]. At the same time that the early versions of this correlation were being asserted, the foundation for a qualitative model predicting the same correlation was being laid by disclosure of the time-varying field-dependent pulsation (now called quasiparallel) structure of the earth's bow shock [Greenstadt et al., 1970; Greenstadt, 1972a]. The resulting model postulated that the time-varying appearance of the daytime micropulsations Pc 3, 4 might be triggered and maintained by the propagation or convection of large-amplitude quasi-parallel shock oscillations through the magnetosheath to the magnetopause [Greenstadt, 1972b]. The model suggested that alignment of Bsw with the solar wind flow, hence essentially with X and therefore parallel to the nominal shock normal at the subsolar point, would furnish the most favorable circumstance for Copyright ¸ 1976 by the American Geophysical Union. transferring quasi-parallel oscillations through the thinnest part of the magnetosheath to the subsolar portion of the magnetopause. Daytime micropulsations would thereby be excited on the magnetospheric field lines rooted at auroral latitudes where Pc 3, 4 have the largest amplitudes. Again, the model suggested the converse as well: namely, that orthogonali...

Section: Data Presentationsupporting

confidence: 68%

Pc 3, 4 activity and interplanetary field orientation

Greenstadt

Olson

1976

“…Therefore Alfv6n waves generated by the garden hose overstability in the plasma sheet should have their magnetic fields polarized highly linearly in the east-west direction. This is in general agreement with the polarization of Pi 1 pulsations observed at Tungsten by Arthur et al [1973] and at College, as shown in Figure 1.…”

Section: Polarization Of Unstable Alfvbn Wavessupporting

confidence: 91%

Generation of irregular (type Pi C) pulsations in the plasma sheet during substorms

Kan¹,

Heacock²

1976

“…Morning Pi(C) ULF waves of the type we are discussing in this paper have not been measured in space [McPherton, 1980]. An intensive effort has been made to find this important class of ground ULF at synchronous orbit without success [Arthur et al, 1973;McPherron, 1975, 1977]. Such studies have shown that Pc 3 are often simultaneous on the ground and at synchronous orbit in the morning sector [McPherron, 1980].…”

Section: Iy = Ex•h = Ex(zh + Am)-ey'(zp + •P)mentioning

confidence: 99%

Detailed correlations of magnetic field and riometer observations at L = 4.2 with pulsating aurora

Arnoldy¹,

Dragoon²,

Cahill³

et al. 1982

Pi 1 magnetic pulsations in the period range 5–15 s recorded on the ground at L = 4.2 during pulsating aurora are studied via the use of data collected at Siple Station, Antarctica, during 1979. Pulsating aurora exceeding 200 R of 4278‐Å emission occurred for approximately 5% of the 1400 hours of photometer operation and was always accompanied by ULF waves. A striking characteristic of the ULF waves is a strong westward component in dB/dt during the auroral light bursts. A detailed correlation of the ULF waves with riometer and flux gate magnetic field data reveal that the magnetic field variations were very closely correlated with D region cosmic noise absorption. Asymmetric pulsations result from the slow recovery of the magnetic field and occurred when the auroral light pulses were of short duration. We conclude from this study that the ground Pi 1 recorded during pulsating aurora at L = 4.2 were due to an overhead current system produced by the particle precipitation. The characteristic westward component in dB/dt could be interpreted as being due to an equatorially directed local ionospheric current driven by a polarization electric field. One must conclude from the results of this paper that the Pi 1 waves studied had no role in the mechanism that precipitated the particles responsible for the pulsating aurora at L = 4.2.