Waves in the Vicinity of the Magnetopause

109

ISEE 1 and 2 magnetic field data obtained during dayside magnetopause crossings have been analyzed using a minimum variance technique in order to determine the characteristics of the rotation of the magnetic field across the magnetopause. Only a small fraction of the magnetopause crossings examined present a sufficiently coherent structure that they can be compared with theoretical predictions, and only a few of the hodograms of the magnetic field at the crossings show the ideal rotation expected for a rotational discontinuity structure. Observed senses of rotation are not consistent with the predictions of the electron whistler polarization theory. Rotations greater than 180° are very seldom observed; they usually occur across multiple crossings, either as 360° rotation about the minimum variance direction or, occasionally, in more complicated patterns associated with ‘S’‐shaped hodograms. These observations suggest that the sense of the magnetic field rotation through the magnetopause is controlled by the relative orientation of the magnetosheath and magnetospheric field and that the angular rotation is minimized when the magnetic field changes from one orientation to the other.

Section: Introductionmentioning

confidence: 99%

Magnetic field rotation through the magnetopause: ISEE 1 and 2 observations

Berchem¹,

Russell²

1982

109

“…Although the magnetopause is known to be very turbulent in the low-frequency MHD portion of the spectrum, relatively little is known about the plasma wave intensities at higher frequencies in the region of primary importance for microscopic plasma processes. Neugebauer et al [1974] and Fairfield [1976] have investigated magnetic measurements of whistler-mode and ion-cyclotron waves near the magnetopause. However, no plasma wave electric field measurements have yet been reported in association with the magnetopause.…”

Section: Introductionmentioning

confidence: 99%

Plasma wave turbulence at the magnetopause: Observations from ISEE 1 and 2

Gurnett

Anderson

Tsurutani³

et al. 1979

183

In this paper we investigate plasma wave electric and magnetic fields in the vicinity of the magnetopause by using recent measurements from the ISEE 1 and 2 spacecraft. Strong electric and magnetic field turbulence is often observed at the magnetopause. The electric field spectrum of this turbulence typically extends over an extremely large frequency range, from less than a few hertz to above 100 kHz, and the magnetic field turbulence typically extends from a few hertz to about 1 kHz. The maximum intensities usually occur in the magnetopause current layer and plasma boundary layer. Somewhat similar turbulence spectra are also sometimes observed in association with flux transfer events and possible ‘inclusions’ of boundary layer plasma in the magnetosphere. In addition to the broad‐band electric and magnetic field turbulence, narrow‐band electrostatic emissions are occasionally observed near the electron plasma frequency in the vicinity of the magnetopause. Two possible plasma instabilities, the electrostatic ion‐cyclotron instability and the lower‐hybrid‐drift instability, are considered the primary candidates for explaining the broad‐band electric field turbulence. The narrow‐band electrostatic emissions near the local electron plasma frequency are believed to be either plasma oscillations or electrostatic waves near the upper‐hybrid‐resonance frequency.

“…They could not determine the amplitude of the wave. Ledley [1971] and Fairfield [1976] carried out similar analyses for other magnetopause crossings. Again, their results depend on an assumed wave speed, and they were unable to determine the wave amplitude.…”

Section: Introductionmentioning

confidence: 99%

Geomagnetopause surface fluctuations observed by Voyager 1

Lepping¹,

Burlaga²

1979

As Voyager 1 moved out of the dawnside of the earth's magnetosphere on September 5, 1977 (at position (−2.6, −16.5, 1.1) earth radii in GSE), it crossed the magnetopause apparently seven times, despite the high spacecraft speed of 11 km/s. Normals to the magnetopause and their associated error cones were well determined for each of the crossings through the use of a minimum variance analysis of the internal magnetic field. The oscillating nature of the ecliptic plane component of these normals indicates that most of the multiple crossings were due to a wavelike surface disturbance moving tailward along the magnetopause. We modeled the wave, which was aperiodic, as a sequence of sine waves with amplitude Ai, wavelength λi, and speed Vi. These quantities were determined for two pairs of intervals from the measured slopes, occurrence times, and relative positions of six magnetopause crossings. The average amplitude was A =2100−500+3800 km, and the wavelengths were of the order of 47,000−12,000+30,000km. The wave speed was approximately 340−95+210 km/s, and typical periods were in the neighborhood of 170±60 s. The magnetopause thickness was estimated to lie in the range 300–700 km with higher values possible. The estimated amplitude of these waves was obviously small compared to their wavelengths; this conclusion is independent of any bulk normal motion of the magnetopause that might have been present.